Microelectromechanical (MEMS) devices are used in a variety of applications including solid-state accelerometers, gyroscopes and pressure sensors. MEMS devices used in these applications often make use of capacitive sensing to determine the displacement of the MEMS structure when it is acted on by various forces. For example, an accelerometer senses the acceleration of the MEMS device along a certain axis of motion. Acceleration causes a displacement of a MEMS proof mass which might be sensed by detecting a change in the capacitance between that proof mass and a fixed electrode. In a pressure sensor, a displacement might be caused by a change in pressure and this change can also be sensed capacitively in some implementations. Gyroscopes make similar use of capacitive sensing elements, in this case to sense displacements related to a rate of rotation of the MEMS device.
When sensing a change in capacitance, a common technique is to place a voltage bias across the capacitor in question so that any capacitance change is indicated by a flow of charge into or out of the capacitor electrodes. This flow of charge may then be amplified and further processed by the sense electronics. A figure of merit of capacitive sensors is the charge sensitivity, which quantifies the amount of charge that flows in response to a known stimulus. It is generally desirable to maximize the charge sensitivity. One very effective means of doing so is to bias the capacitor at a high voltage so that more charge is produced for a given displacement. For this reason, MEMS devices often take advantage of high-voltage biasing.
The use of high-voltage biasing allows for greater sensitivity, which can then be traded for improvements in other system metrics, such as size (cost) and power consumption. However, the use of high-voltage biasing is not without complications. For example, processing high voltages may require the use of special devices and isolation regions that can tolerate high voltage without incurring damage. Such high-voltage devices and isolation regions may not be available in the lowest-cost semiconductor technologies which are typically low-voltage technologies, necessitating the use of more expensive technologies to enable high-voltage biasing and signal processing. The performance gains afforded by the use of high-voltage may therefore be contrary to the goal of cost reduction.
It would therefore be useful to identify techniques for supporting high-voltage biasing in a manner compatible with low-voltage process technologies. The present invention addresses such a need.